The benefits of using an API-first approach in software application design include enabling easy integration with third-party systems, automating common tasks, and providing a solid contract between APIs and their clients, which enables parallel development workflows, improves discoverability, and facilitates reuse. By designing APIs as the foundation of further products and tools, developers can build extensions, integrations, and novel tools more easily. This approach also allows for scalability, flexibility, and reusability, making it easier to power a rich ecosystem of third-party applications in an app marketplace.